8/8/2019 Comparison of Accelerated Corrosion Tests to Corrosion Performance in Natural Atmospheric Environments http://slidepdf.com/reader/full/comparison-of-accelerated-corrosion-tests-to-corrosion-performance-in-natural 1/24 COMPARISON OF ACCELERATED CORROSION TESTS TO CORROSION PERFORMANCE IN NATURAL ATMOSPHERIC ENVIRONMENTS R. Sugamoto University of Hawaii at Manoa Department of Mechanical Engineering [email protected]G.A. Hawthorn University of Hawaii at Manoa Department of Mechanical Engineering [email protected]L.H. Hihara University of Hawaii at Manoa Department of Mechanical Engineering [email protected]ABSTRACT There is interest in the comparison of accelerated corrosion tests to corrosion performance in natural atmospheric environments. Currently, there are some concerns that accelerated corrosion testing may not accurately predict performance in natural atmospheric environments. This provided motivation to compare the corrosion behavior of Al 1060, Al 6061-T6, Al 7075-T6, Al 2024-T3, pure copper, pure magnesium, coated pure magnesium, 1008 steel and pure zinc exposed in a variety of natural atmospheric environments such as rainforest, marine, arid, volcanic and light industrial to a modified GM 9540P 1 cyclic-corrosion test. The modified GM 9540P cyclic-corrosion test was run for 6, 24, 36 and 48 cycles. Results indicated a lack of agreement in the ordering of alloy performance between the accelerated tests and performance in the natural atmospheric environments. Hence, the use of accelerated corrosion testing may not always result in the optimal selection of materials for field use. The data generated also provide the equivalent days of outdoor exposure per cycle of the modified GM 9540P test for all of the alloys. Corrosion rates were also determined as a function of the number of test cycles, showing for which alloys and metals can corrosion rates be predicted with confidence. 1
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Comparison of Accelerated Corrosion Tests to Corrosion Performance in Natural Atmospheric Environments
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8/8/2019 Comparison of Accelerated Corrosion Tests to Corrosion Performance in Natural Atmospheric Environments
There is interest in the comparison of accelerated corrosion tests to corrosion performancein natural atmospheric environments. Currently, there are some concerns that acceleratedcorrosion testing may not accurately predict performance in natural atmospheric environments.This provided motivation to compare the corrosion behavior of Al 1060, Al 6061-T6, Al 7075-T6,Al 2024-T3, pure copper, pure magnesium, coated pure magnesium, 1008 steel and pure zincexposed in a variety of natural atmospheric environments such as rainforest, marine, arid,volcanic and light industrial to a modified GM 9540P1 cyclic-corrosion test. The modified GM
9540P cyclic-corrosion test was run for 6, 24, 36 and 48 cycles. Results indicated a lack of agreement in the ordering of alloy performance between the accelerated tests andperformance in the natural atmospheric environments. Hence, the use of acceleratedcorrosion testing may not always result in the optimal selection of materials for field use. Thedata generated also provide the equivalent days of outdoor exposure per cycle of the modifiedGM 9540P test for all of the alloys. Corrosion rates were also determined as a function of thenumber of test cycles, showing for which alloys and metals can corrosion rates be predictedwith confidence.
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8/8/2019 Comparison of Accelerated Corrosion Tests to Corrosion Performance in Natural Atmospheric Environments
Accelerated tests such as the General Motors 9540P were developed to measure thecosmetic effects of corrosion of automotive painted, or coated, steel surfaces; however, their success at evaluating the performance of these finishes by relative rank has led to wider use.2 It is important, however, to understand the aggressiveness of the accelerated tests and thecorrelation to results obtained by field testing.
In order to gain a better understanding of this correlation, studies were initiated by theHawaii Corrosion Laboratory to compare corrosion data obtained in a wide array of naturalenvironments to those obtained through accelerated testing. Hawaii’s diverse climate providedthe opportunity to collect data in arid, light industrial, marine and rainforest environments.
The coupons exposed outdoors were installed at Hawaii Corrosion Laboratory test sites onOahu, Hawaii. The test sites are representative of light industrial (Campbell Industrial Park),marine (Coconut Island and Kahuku), arid (Ewa Nui and Waipahu) and rainforest (LyonArboretum) environments.
The salt solutions used for testing consisted of the standard GM9540P solution and a 0.5Msodium sulfate (Na2SO4) solution. The sodium sulfate solution was used to determine theeffects of a chloride-free electrolyte versus the standard chloride-containing GM9540P solution.Previous studies conducted at the Hawaii Corrosion Laboratory have compared Al 6061-T6coupled to various ceramic materials exposed at the outdoor test sites to the same specimen
configurations exposed in a humidity chamber, where specimens were first dipped in sodiumsulfate or other chloride-containing electrolytes.3 The results for the sodium sulfate electrolytehad a better correlation to the outdoor studies than the sodium chloride electrolyte. Thestandard GM9540P solution was used for the 6, 24, 36 and 48-cycle tests and the sodiumsulfate solution was used for a 24-cycle test.
EXPERIMENTAL PROCEDURE
Specimens
The specimens (also referred to as “coupons”) chosen for this study include aluminum,copper, magnesium, steel and zinc substrates of various types, finishes and sizes (Figure 1and Table 1).
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L (mm) W (mm) T (mm)Outdoor Test Site Modified GM9540P
The set of 50.8 x 50.8 x 3.2 mm Al 6061-T6 coupons were previously exposed at the same
test sites under similar conditions for 3, 6 and 12 months4 and will be used as a basis for comparison.
Mounting Configuration
Non-conductive Delrin® insulators were used to mount coupons onto portable exposureracks (PERs) that were fabricated from Al 6061-T6. A 316 stainless steel splash guard wasinstalled on each PER to prevent cross contamination of copper ions onto other alloy
specimens (Figure 2).
Atmospheric Test Sites. The PERs were mounted onto support structures for exposure of coupons at a 45-degree angle from the horizontal and oriented to face prevailing winds (North-East).
Cyclic Corrosion Test Chamber. Face plates were mounted at a 45-degree angle from thehorizontal within the chamber.
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Cyclic Corrosion Test Chamber. A Singleton CCT-10 cyclic corrosion test chamber (CCTC) was used to perform the test. Modified versions of the GM9540P cyclic corrosion testwere performed. The modifications refer to the mounting angle being 45-degrees from thehorizontal in all tests and in one test modifications were made to the salt solution.
The salt solutions used for the testing consists of the 1) standard GM9540P solution; 0.9%sodium chloride (NaCl), 0.1% calcium chloride (CaCl2), 0.25% sodium bicarbonate (NaHCO3);and 2) a 0.5M sodium sulfate (Na2SO4) solution.
The standard GM9540P solution was used for the 6, 24, 36 and 48-cycle tests and thesodium sulfate solution was used for a 24-cycle test. The environmental parameters for all of the tests are listed in the table below.
TABLE 4 - List of environmental parameters for the GM9540P test.
FIGURE 3 - Typical temperature and humidity plots for one cycle of the GM9540P test.
Specimen Cleaning and Processing
Coupons were cleaned in accordance with ISO 8407:1991(E)6 except for the steel couponsexposed to modified GM9540P tests. Corrosion products on steel exposed to the 24, 36 and48 cycle modified GM9540P tests were removed using a mild sand blasting process inaccordance with the GM9540P standard. In most cases, several cleanings were required tocompletely remove the corrosion products. After the cleaning process, each coupon wasrinsed, dried and weighed to determine mass loss.
Penetration Rate
After mass loss was determined, calculations for penetration rate were performed.Penetration rates for the 6-month outdoor exposed coupons and the coupons exposed to 6cycles of the modified GM9540P test were based on an average of three coupons. One cycleof the GM9540P test was equated to one day of exposure for purposes of the calculations.
Penetration rates for the coupons exposed to 24, 36 and 48 cycles of the modifiedGM9540P tests were based on an average of two coupons. Only two out of three couponswere available for cleaning and analysis at the time of publication due to delays in X-raydiffraction (XRD) analysis on the third coupon. Once XRD analysis is conducted, the couponswill be cleaned, weighed and added to the data set for penetration rate determination.Therefore, the results should be considered preliminary.
3D Profilometry
Scan Parameters. A MicroPhotonics MicroMeasure 3D non-contact profilometer was usedto obtain 3-dimensional surface maps of the coupons. An optical lens pen with a spot diameter of 25µm and Z (height resolution) of 0.1µm was used for each measurement. The lens penwas used with a scan acquisition rate of 300Hz and lateral step distance of 12.5µm for eachmeasurement. The total scan area was 20 mm x 20 mm, near the center regardless of couponsize.
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Data Processing. After each coupon was scanned, several operators were applied to the
raw data in order to process and extract pertinent information. A “leveling” operator was firstapplied to level the data onto a flat plane.
Five 5 mm x 5 mm areas near the four corners and center of the total (20 mm x 20 mm)
scan area were enlarged to indentify pits. The surfaces were examined for pits larger than20 µm in diameter and deeper than 50 µm.
When pits were identified, an area of 0.5 mm x 0.5 mm was enlarged and a “thresholding”operator was applied to remove surface roughness in the area surrounding the pit in order tomeasure pit depth.
RESULTS and DISCUSSION
Although nine materials were exposed at the outdoor test sites and the modified GM9540Ptest, only a limited number of specimens will be discussed. Future studies will incorporate theentire data set.
TABLE 5 - Available data for comparison. 3 month 6 month 12 month 6 cycle 24 cycle 36 cycle 48 cycle Na2SO4
Aluminum 1060 101.6 50.8 2.0 P, 3D P, 3D P P P P
Aluminum 2024-T3 50.8 25.4 2.0 P, 3D P, 3D P P P P
Aluminum 6061-T6 50.8 25.4 1.5 P, 3D P, 3D P P P P
Aluminum 6061-T6 50.8 50.8 3.2 P P P
Aluminum 7075-T6 50.8 25.4 2.0 P, 3D P, 3D P P P P
Copper CA-110 101.6 50.8 1.7 P, 3D P, 3D P P P P
Magnesium AZ-31B 50.8 25.4 2.4 P, 3D P, 3D P P P P
Magnesium AZ-31B w/ Dow 7conversion coating 50.8 25.4 2.4 P, 3D P, 3D P P P P
Steel 1008 101.6 50.8 1.8 P, 3D P, 3D P P P P
Zinc Grade B6 101.6 50.8 2.5 P, 3D P, 3D P P P P
Exposure
Material Type/Finish/Coating
Coupon Size
L (mm) W (mm) T (mm)Outdoor Test Site Modified GM9540P
P = Penetration rate data available3D = 3D profilometry data available
Corrosion Rate
6-Month Outdoor Corrosion Rates. The table below is a summary of penetration rates for the materials exposed at the outdoor sites.
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Al 7075-T6 0.0019 Al 2024-T3 0.0010 Al 2024-T3 0.0009 Al 2024-T3 0.0010 Al 7075-T6 0.0016 Al 2024-T3 0.0009 Al 7075-T6 0.0764
Zinc
Grade B60.0014 Al 7075-T6 0.0010 Al 7075-T6 0.0007
Zinc
Grade B60.0010 Al 2024-T3 0.0008 Al 7075-T6 0.0007 Al 1060 0.0648
Al 6061-T6 0.0008 Al 6061-T6 0.0003 Al 6061-T6 0.0003 Al 6061-T6 0.0004 Al 6061-T6 0.0003 Al 6061-T6 0.0003 Al 6061-T6 0.0575
Al 1060 0.0007 Al 1060 0.0003 Al 1060 0.0002 Al 1060 0.0003 Al 1060 0.0002 Al 1060 0.0003 Copper CA-110
0.0362
Campbell
Industrial Park
Relative Penetration Rates for Outdoor Test Sites and the 6-cycle Modified GM9540P (mm/yr)
Lyon Arboretum
(Rainforest)
Kahuku
(Marine)
Ewa Nui
(Agricultural)
Coconut Island
(Marine)
6-cycle Modified
GM9540P
Waipahu
(Dry)
HighestRate
Lowest
Rate The steel alloys appeared to behave similarly in the 6-cycle modified GM9540P when
compared to the other materials as did the aluminum. The copper alloys, however, showedsignificantly lower penetration rates relative to the other alloys in the modified GM9540P test.
Comparison Between Accelerated Coupons. The table below presents the materialsexposed to various cycles and solutions of GM9540P, organized by their relative corrosionrates.
TABLE 9 – Relative penetration rate comparison for coupons exposed to the modified GM9540P tests.
The modified GM9540P test using sodium sulfate as the electrolyte produced results whichwere very different from the other GM9540P tests using the standard solution. The penetrationrates overall were significantly lower for the 24-cycle sodium sulfate test when compared to the24-cycle test using the standard GM9540P solution and the relative rankings of copper, zincand Al 2024-T3 were also different.
Comparative Ratios for the Accelerated Tests. Using the steel penetration rates as a basisfor comparison, ratios were calculated to determine which materials performed consistentlythroughout the test sites. Table 10 presents the penetration rate for steel at each site dividedby the penetration rates for the other materials at the same site. Table 11 presents thepenetration rate for steel during each GM9540P test divided by the penetration rates for theother materials exposed to the same test. Steel was chosen as the basis for this comparisonsince the GM9540P test was developed to evaluate painted or coated steel surfaces.
The penetration rate ratios for each material exposed to the outdoor test sites in Table 10were divided by the corresponding ratio for the GM9540P tests in Table 11 to identify whichsites and materials were in line with the accelerated test. The results are presented in Tables12 through 16 below. Ratios that are close to 1.0 indicate materials and test sites that theGM9540P test represents closely based upon comparison to the penetration rates for steel.
TABLE 10 – Ratio of penetration rates for steel to other materials for couponsexposed for 6 months at the outdoor test sites.
Aluminum 1060 44.3 98.3 81.5 101.9 141.3 82.4
Aluminum 2024-T3 10.4 26.2 20.2 28.0 45.6 23.7
Aluminum 6061-T6 39.4 97.8 61.7 83.2 124.9 61.9
Aluminum 7075-T6 16.0 26.7 25.8 20.5 22.6 29.5
Copper CA-110 12.2 8.2 9.4 11.0 18.3 14.2
Magnesium AZ-31B 2.6 1.2 1.4 1.4 0.7 1.7
MagnesiumAZ-31B w/ Dow 7
conversion coating3.6 1.6 2.1 1.9 0.8 2.2
Steel 1008 1.0 1.0 1.0 1.0 1.0 1.0
Zinc Grade B6 21.9 15.2 19.6 30.5 14.6 19.7
RATIO OF STEEL PENETRATION RATE TO OTHER MATERIALS
Material Type/Finish/Coating Campbell
Industrial Park
Coconut Island
(Marine)
Ewa Nui
(Agricultural)
Kahuku
(Marine)
Lyon Arboretum
(Rainforest)Waipahu (Dry)
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With respect to steel, the ratios in Tables 11 through 14 suggest correlation with Al 2024-T3at most of the sites and reasonable correlation for Al 6061-T6 and Al 7075-T6 at some of thesites. The ratios also infer weak or no correlations between the GM9540P test for Al 1060,copper, and magnesium with the Dow 7 chemical conversion coating. The ratios also indicatevery little correlation between the rainforest environment and the GM9540P test. Correlationoverall had a tendency to drop off as the number of GM9540P cycles increased, however it is
important to note that the correlations are also based upon 6 months of exposure at theoutdoor sites.
Penetration Rate Decay Over Time. The penetration rates were plotted 1) as a function of exposure time at six different outdoor test sites for Al 6061-T6 (Figure 4), and 2) as a functionof cycles exposed in the modified GM9540P test for all of the alloys (Figures 5 and 6). Resultswere plotted on a log scale.
The slope (m) of the curves varied from approximately more than -0.1 to less than -1.Ideally, a slope of zero would indicate linear corrosion-rate kinetics; and a slope of -0.5 wouldindicate parabolic corrosion-rate kinetics, where the corrosion rate decreases with time as aprotective corrosion film thickens, slowing the diffusion of ions or migration of electrons. For the set of curves corresponding to the outdoor exposure of Al 6061-T6 (Figure 4), thecorrosion rate would also be dependent on weather parameters which may not be consistentover the exposure period; and hence, the slope of the curve would be influenced by other factors. For Al 6061-T6 at the outdoor sites, the slopes varied from approximately -0.6 to -1.Aside from weather parameters, a hypothesis for slopes less than -0.5 is that corrosion ratesfurther decrease as relatively large cathodic precipitates are undercut and dislodged from themicrostructure, rendering the alloys less prone to corrosion by elimination of cathodic sites.The cases in which the slopes were close to -1 were at Coconut Island and Kahuku wherechloride deposition was relatively high. Interestingly, slopes close to -1 were also observed for Al 6061-T6 and Al 1060 exposed in the modified GM9540P test where their corrosion rates
significantly decreased with exposure cycles in comparison to Al 7075-T6 and Al 2024-T3(Figure 5). The slope for Al 2024-T3 was -0.05, indicating a tendency towards linear corrosion-rate kinetics, which could be due to its relatively high copper content (i.e., 4.4%). The other alloys also showed more tendency towards linear corrosion-rate kinetics: Mg (m = -0.17),coated Mg (m = -0.08), Cu (m = -0.20), Zn (m = -0.26), and 1008 steel (m = -08). It is alsoimportant to note that the R2 values for the aluminum alloys were generally greater than 0.9except for the case of Al 2024-T6 which was very low. The R2 values for the other alloysshowing a tendency towards linear corrosion-rate kinetics were also low, making accurateprediction of corrosion rates by interpolation or extrapolation difficult.
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FIGURE 6 – Penetration rates for copper, steel and zinc exposed to 6, 24, 36 and 48 cycles of the modified GM9540P test (plotted on log scales).
Pit Depth and Pit Density
Aluminum coupons exposed to 6 cycles of the GM9540P test and 6 months at CampbellIndustrial Park (CIP) and Coconut Island (CI) were scanned using a 3D profilometer. Virginspecimens were also scanned as a baseline for comparison.
In most cases, two types of pits were found; pits that were circular in shape and pits thatappeared as trenches. The “trench” pits most likely grew from within scratches on the couponsurface, therefore it is difficult to estimate the true depth of the trench pits. Pits were alsofound on virgin specimens, which were likely to have formed during the cleaning process.
Pits were only considered significant if they were deeper than 50 µm and larger than 20 µmin diameter. “Trench” type pits were not used to compare GM9540P to the outdoor sites sincetheir growth due to corrosion was difficult to estimate.
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1060 aluminum. Only a few pits were found on the 1060 aluminum coupons and themaximum pit depth found on the corroded coupons were similar in depth to pits found on thevirgin specimen.
FIGURE 7 – Examples of pits found on 1060 aluminum.
Al 2024-T3. A significant amount of pits were found on the 2024-T3 coupons compared tothe other alloys. Pits with large diameters and shallow depth were found on the coupons
exposed to GM9540P and at Campbell Industrial Park. Most of the deeper pits were small indiameter.
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Al 6061-T6. All of the pits found on the GM9540P coupons were trench type pits and werenot used for comparison. There were few pits found on the other coupons. The pits that werefound were considerably deep, however, similar pits were also found on the virgin specimen.
Al 7075-T6. All of the pits found on the GM9540P coupons were trench type pits and werenot used for comparison. A very deep pit was found on the coupon exposed at the CoconutIsland site, roughly 3 times deeper than those found on the virgin specimen.
The penetration rates for specimens exposed at the outdoor test sites generally followed atrend, however, when the relative rates were compared to the modified GM9540P tests,several materials did not follow the same trend.
When considering the relative penetration rates in Table 8 (6-month outdoor and 6-cycleGM9540P tests), copper and zinc in particular did not seem to follow the same trend relative tothe other materials. With Lyon Arboretum as the exception, the penetration rate for copper
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The authors would like to acknowledge the following for their contributions:
• Mr. Robert Zanowicz, US Army ARDEC, Contract Officer’s Representative and ProgramManager for the Pacific Rim Corrosion Research Program and Pacific Rim
Environmental Degradation of Materials Research Program
• Mr. Lance Miyahara, Hawaiian Electric Company.
• Mr. Alex Niemi, UH Manoa (MS 2005).
REFERENCES
1. General Motors Engineering Standards, Accelerated Corrosion Test, GM9540P, Dec,1997.
2. Raymund Singleton, “Cabinet Testing,” in ASM Handbook Volume 13A Corrosion:Fundamentals, Testing, and Protection, ASM International, 2003, p. 470
3. R. Srinivasan, "Corrosion Studies Between Ceramics and 6061-T6 Aluminum Interface,"M.S. Thesis, University of Hawaii at Manoa, 2005.
4. G.A. Hawthorn, M. Nullet, R. Srinivasan, L.H. Hihara, “Corrosion Testing andAtmospheric Monitoring in an Active Volcanic Environment,” Tri Services Corrosion
Conference, Denver, Colorado, 2007.5. G.A. Hawthorn, L.H. Hihara, “Corrosivity Mapping of the Pacific Theater of Operations,”
NACE 2008 , New Orleans, Louisiana, 2008.6. ISO 8407:1991(E) Corrosion of Metals and Alloys – Removal of Corrosion Products